The present invention relates to a method for manufacturing workpieces or blocks from meltable materials, where the material, starting as a liquid is solidified in a directed manner in a casting mold by using a cooling device.
The invention additionally relates to a device for manufacturing workpieces or blocks from meltable materials using a casting mold that is heatable using a heating device and where a cooling device is assigned to the base of the casting mold.
Subsumed under the term xe2x80x9cmeltable materialsxe2x80x9d, as it is used here, are materials made of ceramics, including sapphires, rubies, spinels, etc., metals, metal alloys, or materials from the group of semiconductors with an oriented multi-crystalline or mono-crystalline structure.
With such methods as relate to the invention, as well as the respective devices, the starting material is either provided to the casting mold in the liquid phase or is melted in the casting mold and thereafter solidified in a directed manner in the casting mold.
Such a type of solidification guiding is known in various prior known methods. According to one method, or the corresponding device, respectively, described in GB-A-2 279 585, the casting mold with the melt is pulled downwards out of a heating furnace. This results in the solidification front progressing from bottom to top. For long components or materials with a low thermal conductivity, the influence of an applied cooling plate becomes insignificant after just a few centimeters. After that, heat removal occurs basically on the sides via the chill surface, which in practical applications does not enable setting an essentially plane phase boundary between already solidified and liquid melt material. This method is not suitable for manufacturing large-surface blocks solidified in a directed manner because with large cross-sections, the heat conducting paths from the center of the block to the heat removing surfaces on the sides are too long making it impossible to achieve plane phase boundaries in connection with sufficiently high temperature gradients.
In the Technical Digest of the International PVSEC-9, Miyazaki, Japan 1996, Ritsua Kawamura et al. show in xe2x80x9cRecent Progress in Electromagnetic Casting for Polycrystalline Silicone Ingotsxe2x80x9d that the phase boundary between solid and liquid silicon has a significant concave shape. Parallel radial crystalline structures cannot be achieved using this method. The maximum block size is described as 22 cmxc3x9722 cm.
Larger silicon blocks solidified in a directed manner are manufactured according to the state-of-the-art in blocks of 66 cmxc3x9766 cm and a height of 2.5 cm using the HEM Method (Heat Exchanger Method). With the HEM Method, according to the state-of-the-art, the energy required to maintain the solidification speed and the temperature gradient is removed across a central area of the chill bottom. With a constant temperature of the heater located above the melt surface, the heat transfer coefficient between the base of the chill and the cooling plate essentially determines the heat removal flow, and thus, the speed of growth of the crystalline block.
It is the objective of the present invention, beginning with the state-of-the-art described above, to develop a method and device with the features mentioned above, such that the solidification of the melt can be guided in a defined manner and where the initiation of the cooling phase can be continuously advanced from the heating phase to the cooling phase. In addition, the device and the method shall offer a broad variation with simple means of design with regard to this definably guided solidification in a defined manner.
This objective is accomplished with the above mentioned method such that a cooling structure with at least one thermal conductor is inserted in a least one corresponding recess from the bottom side into the body assigned to the base of the casting mold in order to accomplish a defined guiding of the solidification front during the cooling phase of the molten material.
With regard to a device, the objective is accomplished in that the above mentioned device is characterized in that the cooling device contains a cooling structure with at least one thermal conductor that can be inserted from the bottom into at least one corresponding recess in a body assigned to the base using a sliding mechanism.
Using the described method and device, the solidification of the material starting as a liquid that was poured into the casting mold can be carried out in a defined guided manner beginning at the base of the casting mold by inserting the thermal conductor in different positions into the recess of the body assigned to the base of the casting mold. By adjusting the at least one thermal conductor in the at least one recess assigned to this body, the heat transfer, and thus the cooling performance, can be set in a defined manner and can be changed as well. Through the respective geometry of the thermal conductor and its corresponding recess, it is additionally possible to influence the solidification front that moves from the base upwards. Depending on the number of thermal conductors and the corresponding recesses of the used cooling structure, crystallization speeds of 0.2 mm/min to 2 mm/min can be achieved with cooling performances in a range from 10 to 150 kW per m2.
In order to change the amount of removed heat per unit of time in addition to adjusting the thermal conductor in the corresponding recess, it may also be advantageous to maintain a gaseous atmosphere with an adjustable pressure surrounding the cooling structure. By lowering the gas pressure to a few mbar, the performance density can be controlled more sensitively. Furthermore, a gaseous atmosphere of Argon should be maintained around the heat conducting body in such a case, where continued purging is carried out with such a gas because, especially with Argon, additional contamination can be removed from the heating space.
As already indicated, the cooling structure may contain several thermal conductors that can be inserted in slots and/or blind holes of the body that is assigned to the base of the casting mold. Suitable thermal conductors are plates, bolts and/or bars that may additionally be designed with different cross-sectional geometries. In an embodiment that should be emphasized especially, a heating device is positioned underneath the base of the casting mold such that the one or more thermal conductors in the inserted position penetrate through the heating device into the body that is assigned to the bottom side of the base. With such an arrangement, the transition between heating and cooling of the casting mold can not only be determined by the insertion of the thermal conductors into the recess(es), but also through additional control of the heating device, since it also essential to heat the base of the casting mold for maintaining the liquid phase of the starting material. For this purpose, the heating device may be placed in a carrier plate that is assigned to the base of the casting mold and that carries the casting mold. The carrier plate is then provided with holes or recesses that serve the purpose of changing the outer surface that is available overall for heat transfer in a broader range than would be possible with the bottom surface of the base of the casting mold alone.
Preferred dimensions of such thermal conductors are diameters or thicknesses and/or width of 5 mm to 20 mm, preferably of 10 mm to 14 mm. Furthermore, the width of the web remaining between neighboring recesses in the body in which the thermal conductors are inserted should be between 5 and 20 mm. Furthermore, the insertion depth of the thermal conductors into the body should be at least 20 mm to be able to adjust the cooling performance over a sufficient range. However, the individual thermal conductors may have a significantly greater length than corresponds to an insertion depth of 50 mm, that is, the height of the thermal conductor may be between 100 and 150 mm, preferably about 130 mm.
In the simplest case, the thermal conductors are designed as round pins. For reasons of stability, the diameter of such a thermal conductor in its design as a round bolt should not be selected to be less than 10 mm. The ratio between the effective exchange surface and the flat surface with a remaining web width of 10 mm with a bolt diameter range of 10 mm to 20 mm is practically independent of the selected bolt diameter. To further increase the cooling performance, the individual thermal conductors may have a cross section in the shape of a cross or star. Such thermal conductors then enter recesses in the body assigned to the base of the casting mold where the shape of the cross-section of said recesses is adapted to the shape of the element, such that large areas are provided both in the recesses and on the cooling elements. To have as large a range of cooling performance as possible, with the cooling power being adjustable, the ratio of the sum of the cross-sectional areas of the thermal conductors to the sum of the cross-sectional areas of the recesses should be between 1.5:1 and 5.5:1. This results in possible cooling powers of about 10 to 150 kW/m2.
The movement of the thermal conductors in the recesses of the body that is assigned to the casting mold can be easily realized, technically, with a lifting mechanism. With a stroke of 50 mm and a thermal conductor made of copper with a diameter of 12 mm and an effective height of the thermal conductor of 130 mm and a hole distance of 26 mm and a hole diameter of 14 mm, a heat transfer coefficient of about 10 W/(m2xc3x97K) to about 240 W/(m2xc3x97K) can be set for a 1000 mbar Argon atmosphere between carrier plate and thermal conductor at a carrier plate temperature of 1400xc2x0 C. These values correspond to about 1400 to 1500 thermal conductors per square meter.
The thermal loss through the thermal insulation is negligible due to the small ratio of diameter to hole length, such that, with a retracted cooling structure, the thermal losses through the open penetrations are tolerable.
It is further possible to control the removed energy density even more sensitively by reducing the gas pressure to a few mbar. For this purpose, the entire cooling structure can be arranged in a chamber with an adjustable pressure. For effective heat removal, it is particularly advantageous if the body is an integral part of the base of the casting mold and furthermore, if this base is textured, for example with elevations and depressions, where the respective thermal conductors are moved into or are retracted from the bottom in respective holes in the elevations of the base of the casting mold.
As already mentioned above, the device subject to the invention enables setting a temperature profile directly above the chill or casting mold base surface. Through this special design of the chill or casting mold base, the radial crystallization can be influenced in the area of the depressions, viewed from the base area. The lowest points of these individual depressions are aligned with the respective thermal conductors such that crystallization starts at the lowest (coldest) points of the chill base. In this manner, a slightly planar or slightly convex phase boundary between solid and liquid material can be set intentionally to achieve a certain objective, for example, to initiate a thermal convection. Research has shown that, especially when taking the objective of cleaning into account, a slightly curved phase boundary is advantageous in directed (controlled) solidification.
For a full understanding of the present invention, reference should now be made to the following detailed description of the preferred embodiments of the invention as illustrated in the accompanying drawings.